Because of increasing congestion in the currently used microwave spectrum (3-30 GHz) for space-to-Earth communications there is a study underway to develop the multi-GHz bandwidth available at millimeter-wave frequencies above 30 GHz. The millimeter-wave frequency bands of interest include Q-band (37-43 GHz) and E-band (71-76 GHz and 81-86 GHz). Prior to the use of these millimeter-wave frequency bands it is necessary to characterize the many atmospheric effects, including rainfall, cloud coverage and gaseous adsorption on millimeter-wave signals. For high data rate wide band millimeter-wave communication systems it is also necessary to characterize the frequency-dependent group delay effects. The design of any operational millimeter-wave communication system will depend on the results of the propagation studies.
However, in conventional low frequency satellite borne microwave beacon transmitter used in the investigation of Earth's atmosphere on radio wave propagation from space, the frequency of the beacon is fixed at a single frequency. A major limitation of conventional beacon transmitter or approach is that it assumes that the atmospheric attenuation and group delay effects are constant at all frequencies across the narrow frequency band of interest. However, at millimeter-wave frequencies the atmospheric attenuation and group delay effects are not constant at all frequencies across the band of interest. Hence, measurements have to be made at multiple frequencies across the entire multi-GHz wide frequency band of interest. Sequentially stepping the frequency of the beacon transmitter in small discrete increments is also not an option, since in the time interval between frequency increments, transmission, and measurements; the characteristics of the atmosphere could drastically change. Hence, in the new tunable satellite borne beacon transmitter, which is based on the multi-tone multi-band millimeter-wave synthesizer described here, multiple frequency tones that are coherent and representative of the desired Q-band and E-band frequencies of interest are simultaneously generated and transmitted. Thus, the new beacon transmitter enables simultaneous, fast, more accurate wide band characterization of radio wave propagation from space through the Earth's atmosphere. Besides radio wave propagation studies, the multi-tone frequency synthesizer can be used for space borne active remote sensors like scatterometers, which require coherent, highly stable multi frequency signals.